Processes for producing vinyl acetate by the vapor phase reaction of ethylene, acetic acid and oxygen using supported catalysts containing metallic palladium, metallic gold and an alkali metal acetate are known and disclosed in U.S. Pat. Nos. 3,743,607 and 3,775,342.
Various procedures are used to prepare the supported palladium-gold catalysts employed for these processes. All of the procedures generally include a reduction step. Since the metals are typically deposited on the support from aqueous metal salt solutions, they must subsequently be reduced to the corresponding metallic state by treating the support having the metal salts deposited thereon with a reducing agent, such as ethylene, propylene, isobutylene, hydrazine, hydrogen, methanol, formaldehyde, formic acid and salts, and the like. After the reduction, the support having substantially all of the palladium and gold present in the metallic form is activated by impregnating with an alkali metal acetate, such as sodium or potassium acetate.
In several instances, an oxidation step is also included in the catalyst preparation procedure. In U.S. Pat. No. 5,194,417, for example, after substantially all of the palladium and gold have been reduced to the metal, the "virgin catalyst" is oxidized or at least partially oxidized by contacting with an oxidizing agent, preferably an oxygen-containing gas, and then after removal of the oxidizing agent contacted with a reducing agent. By such treatment of "virgin catalyst," improvements in selectivity and yield are achieved.
Another process which, in one embodiment, includes both oxidation and reduction steps in the preparation of a palladium-gold supported catalyst is disclosed in U.S. Pat. No. 4,087,622. In this process, prior to deposition of the major proportion of the palladium and gold, a small amount of metal salt is deposited on the support, converted to the corresponding metal oxide and then subsequently reduced to the metal. This sequential oxidation and reduction of metal salts to achieve predeposition of small amounts of metal on the support before deposition of the major portion of palladium and gold is distinguished from the present invention where all of the metal salt is deposited onto the support and the impregnated support heated in a non-reducing atmosphere before reducing.
U.S. Pat. No. 3,917,676 discloses palladium-lead catalysts for the reaction of propylene and acetic acid with oxygen to produce allylacetate. In one embodiment, after deposition of the metal salts and drying, the palladium-lead catalyst is sintered in air at 250.degree. C. for 5 hours and then subjected to hydrogen reduction at 200.degree. C. for 5 hours.